The present invention relates to a resistor array circuit device and a variable gain device utilizing the same for audio equipment and the like.
FIG. 3 is a prior art electrical circuit diagram showing a resistor array circuit device and a variable gain device utilizing the same.
In the prior art circuit shown in FIG. 3, a resistor array circuit device is constructed by an n stage R-2R ladder resistor array, n switches SW and a control circuit CNT for controlling the states of the n switches SW. The variable gain device is constructed by this resistor array circuit device, an operational amplifier OP and a feedback resistor of the operational amplifier OP. Since the iterative impedance value of the R-2R ladder resistor array is "R" and the value of the feedback resistor is also "R", this variable gain device functions as a variable attenuator. In other words, when all of the n switches SW are connected to the inverted input terminal side of operational amplifier OP (which occurs when the control signal or digital code from control circuit CNT is "111 . . . 11"), the attenuation factor is 1. The attenuation factor or attenuation amount is changed in a stepwise manner by modifying the digital code. It is noted that the input side of the variable gain device is the MSB and the operational amplifier side is the LSB.
Japanese Laid-open Publication No. S64-61109, and specifically FIG. 4 thereof, is an example of such a prior art circuit.
In the above prior art circuit, modification of the digital code and switching of switches SW are performed simultaneously. At this time, if there are bits which are modified from "0" to "1" there are also, conversely, bits which are modified from "1" to "0". Since modification of this type of digital code is principally bidirectional, in a transitional state where the switches SW are simultaneously switched, there is a problem that spike-shaped noise, which is known as gridge noise and arises from a difference in the on speeds and off speeds of switches SW, occurs.
Also, in the above prior art circuit, precise modification of the digital codes such that stepwise modification of the attenuation factor progresses geometrically, that is, progresses arithmetically in the case of a logarithmic expression, is troublesome for the following reasons. Where the attenuation factor is within a small range, for example, the range wherein the digital code is from "111 . . . 11" to "011 . . . 11", since many digital codes within this range are selectable, relatively precise digital codes along the target geometric progression can be obtained. However, where the range of the attenuation factor is large, for example where the range of the digital code is from "00 . . . 011" to "00 . . . 001", the selectable digital codes are few, and it is difficult to obtain precise digital codes along the target geometric progression. In other words, it is difficult to precisely modify the digital codes across all ranges.
The two above-described problems do not only occur where the variable gain device shown in FIG. 3 is used as a variable attenuation device, but also where it is used as a variable amplification device.